This invention relates generally to radiotelephones and particularly to reducing battery consumption in a radiotelephone when no network signal is found during an initial synchronization attempt.
It is desirable in radiotelephones to minimize battery consumption in order to maximize the time required between recharging radiotelephones. When a radiotelephone is initially powered-up, it is necessary to scan all of the operative channels to obtain a signal from the base station and perform an initial synchronization of the radiotelephone. If a sufficiently strong signal to perform the initial synchronization is not found, the synchronization cannot be accomplished and the radiotelephone cannot be used. Therefore, the radiotelephone must continue to scan all channels until a sufficiently strong signal from the base station is received. However, this continual scanning results in large power consumption of battery power, and greatly shortens the time a radiotelephone may be used before it must be charged again.
In the past, attempts to minimize battery consumption during initial synchronization attempts have included incorporating a delay in synchronization attempts when it is determined that the cellular phone is out-of-range of a base station and/or turning the radio telephone on and off when the radio telephone is out-of-range.
For example, in U.S. Pat. No. 5,375,254 (xe2x80x9cthe ""254 patentxe2x80x9d), to conserve battery power, instead of continuously rescanning all channels to obtain initial synchronization when the radio telephone is out-of-range, the successive rescans are made less frequently as the time interval from when the handset was last in-range increases, until a predetermined maximum interval has been reached between successive rescans. Thus the ""254 patent reduces battery consumption by adaptively reducing the frequency of attempting to find a channel for initial synchronization. The interval between rescans in the ""254 patent is not a function of the signal strength measured on a channel, but rather a function of the time since a valid signal was last acquired. The disadvantages of increasing the interval between rescans is that the handset will not respond rapidly to appearance of a signal.
In U.S. Pat. No. 4,903,319 (xe2x80x9cthe ""319 patentxe2x80x9d), a battery-powered portable radio telephone for use in a mobile telephone network receives signals from the network and determines from the received signal whether the radio telephone is within the range of the service area of the network. A battery saver is provided to periodically interrupt the battery power of the telephone when the radiotelephone is out of range of the service area. Thus when no valid signal is detected, the handset is switched on and off in a periodic duty cycle to conserve power. The ""319 patent suffers from the same disadvantage as the ""254 patent in that to achieve sufficient reduction in battery consumption, the time between successive re-powering of the radiotelephone should be much longer than the time to obtain initial synchronization. Therefore, a radiotelephone comprising the ""319 patent will be slow to respond to the appearance of a signal.
The increasing availability of cellular spectrum has increased the complexity of the sequence the phone must execute to rapidly find a signal with which it is compatible. The channels of this spectrum are used by various operators in the U.S.A. to transmit different, incompatible signals. These signals include cellular telephone signals for systems such as the Advanced Mobile Phone System (AMPS); Digital AMPS (D-AMPS); Global System for Mobile Communication (GSM or, in the United States, PCS1900); and Code Division Multiple Access (CDMA) system. Because of the increased channel space in these systems, the process of finding and synchronizing to a control channel must be improved to reduce the battery power required for this function.
A radiotelephone according to one embodiment of the present invention is designed to operate in a mobile communication system such as a Code Division Multiple Access (CDMA) system, a Time Division Multiple Access (TDMA) system, or other systems presently known or developed in the future. A network station transmits traffic signals to individual radiotelephones engaged in communications via the CDMA system as well as broadcast information intended for radiotelephones not presently engaged in communications via the system. The broadcast information contains known signal patterns for use by radiotelephones at power-on to acquire code-lock synchronization with the broadcast information.
When a radiotelephone fails to acquire synchronization upon power-up after a predetermined time, the radiotelephone will, according to one embodiment, characterize and electronically memorize the radio environment in which it failed to acquire synchronization. In this embodiment, the memorized characteristic of the radio environment includes at least the total received signal strength in each radio frequency channel upon which synchronization was attempted. The radiotelephone then enters a low power mode in which only a low-power clock-timer circuit is active to re-power the radiotelephone at periodic intervals to re-characterize the radio environment.
After re-characterizing the radio environment, the radiotelephone then detects whether the radio environment has changed by more than a predetermined threshold. If the radio environment has changed by more than the predetermined threshold, the radiotelephone will re-attempt initial synchronization.
In one embodiment, the clock-timer circuit causes the radio telephone to re-measure the total received signal strength in at least one of the previously tested frequency channels after a given time, for example, after one second. The radio frequency environment is characterized by averaging over a time that is greater than the reciprocal of the receiver bandwidth by about two orders of magnitude, which, for a receiver bandwidth of 1 to 5 MHz is still only about 100 uS. Such averaging allows a measurement accuracy of a fraction of a decibel. As a result, the duty factor of making repeat measurements of signal strength is only 1/10000, leading to very low average battery consumption during a prolonged power-up where the signal strength is not great enough for initial synchronization. Accordingly, unlike previous devices, the illustrated embodiment does not deplete the battery in making continuous attempts to synchronize with a network station.
The illustrated embodiment only makes a new attempt to synchronize if the average signal strength changes by a predetermined amount, for example, by 3 dB, indicating appearance of a signal or disappearance of interference. The channel upon which the illustrated embodiment makes repeat measurements can be cyclically varied through the list of radio channels on which initial synchronization failed. If a second subsequent attempt to acquire synchronization on a given radio channel also fails, the radio environment on that channel will be updated in memory, so that a reattempt will not be made until the environment changes once more from the updated value to a new value differing by more than a threshold amount.
In one embodiment, synchronization is attempted with sufficient frequency to rapidly detect the appearance of a signal, but also reduces battery consumption by keeping the power-on time to a minimum by making only a short signal strength measurement, rather than a full synchronization re-attempt. The signal strength measurement is compared with the signal strength measured on that channel and stored during the initial, unsuccessful synchronization attempt and a full new synchronization attempt is not made unless the new signal measurement exceeds the stored signal measurement by a predetermined threshold.
Alternate embodiments can include other features such as searching for initial synchronization on frequency channels in a prioritized order or using information about the channels on which synchronization has been acquired most recently. An initial synchronization attempt may also commence by making a rapid determination of signal strength on all channels to be searched and making synchronization attempts on those frequency channels having the greatest signal plus noise power content. Synchronization attempts may also include searching in a reduced bandwidth for an unmodulated signal burst that occurs with known frequency (also known as the power profile method) in order to acquire coarse time and frequency synchronization prior to searching for fine synchronization by correlation of shifts of the received signal with a known code or signal pattern.